1. Field of the Invention
The present invention relates to the field of well drilling and completion. More specifically, the present invention relates to direct measurement apparatus and methods for evaluating subsurface conditions in a wellbore.
2. Description of the Background Art
In a typical well drilling operation, conditions in the wellbore must be closely monitored and controlled to optimize the well operation and to maintain control of the well. One of the most important conditions in well drilling procedures is the bottomhole pressure of the circulating drilling fluid or xe2x80x9cmudxe2x80x9d used in forming or conditioning the well. The actual or effective density of the mud is an important condition that can be affected by a number of different variables related to the composition of the mud, the characteristics of the formation being penetrated by the wellbore, the dynamics of the drilling mechanism, and the procedures being implemented in the wellbore. In this latter regard, for example, the circulation of the fluid creates an effective density within the wellbore, referred to as an equivalent circulating density, that exceeds the static density of the fluid. The equivalent circulating density is caused by pressure losses in the annulus between the drilling assembly and the wellbore and is strongly dependent on the annular geometry, mud hydraulics, and flow properties of the well fluid. The maximum equivalent circulating density is always at the drill bit, and pressures of more than 100 psi above the static mud weight may occur in long, extended reach and horizontal wells.
This equivalent circulating density, which must be known in order to determine well pressures existing at different locations within the wellbore, may be calculated using hydraulics models from input well geometry, mud density, mud rheology, and flow properties, through each component of the circulating system. There are, however, often large discrepancies between the measured and calculated pressures due to uncertainties in the calculations through poor knowledge of pressure losses through certain components of the circulation system, changes in the mud density and rheology with temperature and pressure, and/or poor application of hydraulics models for different mud systems.
In many high pressure, high temperature (HPHT), deepwater, and extended reach and horizontal wells, the margin between the formation pore or collapse pressure and the formation fracture pressure often diminishes to the point that the equivalent circulating density can become critical. In extreme cases, the well may flow or cave in while the pumps used to circulate the mud are off (xe2x80x9cpumps offxe2x80x9d), allowing the well fluid to flow into the formation. Accurate determination of the actual static and dynamic mud pressures within the wellbore is therefore a critical design parameter for the successful drilling of these wells.
Another phenomenon affecting pressures in the wellbore results from movement of the drill string. As the drill string is lowered into the well, mud flows up the annulus between the string and the wellbore and is forced out of the flowline at the well surface. A surge pressure results from this movement, producing a higher effective mud weight that has the potential to fracture the formation. A swabbing pressure occurs when the pipe is pulled from the well, causing mud to flow down the annulus to fill the void left by the pipe. The pressure effectively reduces the mud weight and presents the potential for inducing a discharge of fluid from the formation into the wellbore. As with the equivalent circulating density measurements, the swab and surge pressures are strongly dependent on the running speed, pipe geometry, and mud rheology involved in the drilling or completion of the well. These pressures reach a maximum value around the bottom hole assembly (BHA), where the annular volume between the drilling string assembly and the surrounding wellbore is the lowest, and thus where flow through the well is the fastest.
Theoretical and experimental evidence suggests that during running pipe in and out of the wellbore, a much larger pressure differential is exerted on the formation than is experienced from static and circulating pressures during drilling, unless the pipe running speed is lowered significantly. Formation susceptibility to wellbore instability, although not problematic while drilling, may increase due to the swab and surge pressures incurred during tripping when the entire pipe string is rapidly withdrawn or reinserted in the well.
Modeling swab and surge pressure is difficult because of the manner in which the fluid flows as the pipe is moved within the well. A moving pipe causes the mud adjacent to the pipe to be dragged with it to a certain extent, although the bulk of the annular fluid is moving in the opposite direction. The mechanics are therefore different from the hydraulics calculations described for the mud circulation since, in that case, fluid flow is considered to be only moving in one direction. Swab and surge hydraulics models therefore require a xe2x80x9cclinging constantxe2x80x9d to account for the two relative motions.
A pressure surge caused by breaking the gels when increasing the flow rate too quickly after breaking circulation has been responsible for many packoff and lost circulation incidents. In this situation, where the well circulation is terminated for a period of time (xe2x80x9cpumps offxe2x80x9d) and then reinitiated (xe2x80x9cpumps onxe2x80x9d), if the circulation rate is reinitiated too quickly, a pressure surge is created in the mud, causing a damaging imbalance with the formation. This danger, which is particularly evident in high angle wells, led to the procedure of slowly bringing the volume of the mud pumps up anytime after circulation is temporarily suspended. A pressure surge associated with restarting circulation may also be caused by a restriction in the annulus due to cuttings sagging and accumulating while the mud is static.
In extended reach and horizontal wells, hole cleaning can become critical. If parts of the wellbore are unstable, as in common in these types of wells, the accumulation of cuttings, beds, and an overloaded annulus make it difficult to clean the hole properly. Remedial measures, such as control drilling, the pumping of viscous pills, and wiper trips, are commonly employed in an attempt to avoid packing off and sticking the pipe. These procedures, however, consume valuable time and may also damage the formation leading to further wellbore instabilities.
Yet another situation where knowledge about the subsurface conditions is important occurs when drilling out of the bottom of a casing shoe into new formation. It is common to perform a leak-off test (LOT) to determine the strength of the cement bond around the casing shoe. However, because of the small margins between the formation pore or collapse pressure and fracture pressure in many wells, the LOT has become a critical measure of the formation strength and is used as a guide to the maximum allowable circulating pressure that may be used in a subsequent hole section without breaking down the formation and losing circulation in the well.
Conventionally, LOT pressures are recorded at the surface of the well. The measurements must be corrected for the pressure being exerted by the mud column. To obtain an accurate reading in these surface conducted measurement procedures, the mud must be circulated thoroughly to condition it to produce an exact and even density for the LOT calculation. This process can be time-consuming, and the calculated results are subject to the correctness of the information and assumptions used for the values of the variable conditions affecting the mud column density.
Subsurface pressure information is especially important when the well xe2x80x9ctakes a kickxe2x80x9d during drilling. The term xe2x80x9ckickxe2x80x9d is commonly employed to describe the introduction of formation gas, a lower density formation fluid, or a pressured formation fluid into the wellbore. If not controlled, the kick can reduce the density of the drilling fluid sufficiently to allow the formation pressure to flow uncontrollably through the well and become a xe2x80x9cblowout.xe2x80x9d In riserless offshore drilling, the kick can allow formation fluids to flow into the sea.
After the kick is detected and the well is shut in, the stabilized casing shut-in pressure and the stabilized drill pipe shut-in pressure are measured at the well surface and recorded. The drill pipe shut-in pressure is used as a guide in determining the formation properties. Since the formation fluid type is generally unknown, it is not possible to determine the formation pressure from the casing shut-in pressure. The formation pressure and influx volume are required to calculate the density of the mud required to xe2x80x9ckillxe2x80x9d the well. While circulating the kill mud, the annular pressure is controlled by the choke and pump speed to maintain a constant bottom hole formation pressure and prevent further entry of formation fluid. As with the other evaluations dependent upon fluid or mud pressure, the accuracy of the calculations is dependent upon the correct evaluation of the factors affecting the mud density.
Another situation that requires knowledge of the mud column density is that of determining the mud weight. The mud weight is normally determined at the well surface from surface mud checks or sensors in the flowline or the return pit. It has been proposed that the mud density actually decreases with temperature increases due to expansion and that this effect may become important in HPHT wells with tight margins between the formation pressure and the wellbore pressures. In high angle wells, a heavy cuttings load may increase the annular mud weight significantly. Additionally, a number of measurements can be made during a trip to detect barite sag, which also affects the mud weight.
A conventional pressure while drilling (PWD) tool can be used to measure the differential well fluid pressure in the annulus between the tool and the wellbore while drilling mud is being circulated in the well. These measurements are employed primarily to provide real-time data at the well surface, indicative of the pressure drop across the BHA for monitoring motor and measurement while drilling (MWD) performance. The measurement values are also affected by the effects of the circulating well fluid. Direct annular pressure measurements were not customarily made.
Downhole well pressures may also be measured directly using a drill-string-supported tool isolating a section of the wellbore from the effects of the well fluid above the point of measurement. U.S. Pat. No. 5,555,945 (the ""945 patent) describes a tool that employs an inflatable packer with an MWD instrument designed to sense fluid pressure or temperature, or other variable well characteristics. The measurement is typically made in the annulus between the tool and the formation in the area below the set packer. The packer is set and the subsurface variable is measured and recorded in an instrument contained within an assembly of the tool. The recorded data is retrieved to the surface by pulling the drill string and assembly from the well. Constant remote communication may be maintained with a surface command station using mud pulse telemetry or other remote communication systems.
U.S. Pat. No. 5,655,607 describes a drill-string-supported, inflatable packer that can be anchored in an open wellbore and used to measure well pressures above or below the packer. An internal cable control is used to regulate inflation and deflation of the packer. Subsurface measurement data are presumably sent directly through the cable to the well surface or recorded and retrieved when the assembly is retrieved to the well surface.
In some MWD systems, downhole temperature and pressure, as well as other parameters, are measured directly, and the measured data values are communicated to the surface as the measurements are being made using xe2x80x9cfluid pulse telemetryxe2x80x9d (FPT), also called xe2x80x9cmud pulse telemetryxe2x80x9d (MPT). FPT, such as described in U.S. Pat. No. 4,535,429, requires that the well fluid be circulated to transmit data to the well surface. While data transmission during circulation of the well provides information on a timely basis, the measurements taken are affected by the fluid circulation and must be corrected for its effects. This requirement imposes the same uncertainties previously noted regarding calculated values for subsurface parameters, computer modeling, and surface measurement techniques used to estimate a subsurface condition.
It is also possible to directly obtain subsurface measured data using transmission techniques that do not rely on circulating well fluid. For example, subsurface measurement and transmitting devices using low frequency electromagnetic waves transmitted through the earth to a receiver at the surface are capable of transmitting data without regard to whether the well fluid is circulating or static. These devices, however, are not suitable for use in all applications and also require highly specialized transmitting and receiving systems that are not as commonly available as are the FPT systems.
MWD systems that use MPT are only able to send information to the surface while circulating. Thus, real-time pressure and temperature information can only be sent real time while circulating the mud system. However, much information useful to well drilling and formation evaluation processes can be gained from the data recorded while the pumps are off. While the pumps are off, pressure and temperature and other data are recorded at a specific sampling rate. On resumption of circulation, this stored information is transmitted to the surface using FPT. This may be as detailed as each discrete recorded sample. However, sending all data may take an unacceptable amount of time. Some smart processing downhole will reduce the amount of data that has to be sent up.
U.S. Pat. No. 4,216,536 (the ""536 patent) describes a system that, among other things, uses the storage capacity in a subsurface assembly to store data measurements of a downhole condition made while the drilling liquid is not circulating. The stored data is transmitted to the well surface after flow of the drilling liquid is resumed using FPT. Subsurface temperature and formation electrical resistivity are examples of the conditions sensed and recorded while the circulation of the drilling fluid is interrupted. The ""536 patent also discloses a method for increasing the effective transmission rate of data through FPT by deriving and transmitting condensed data values for the measured conditions. The ""536 patent employs multiple transducers on a logging tool for measuring a number of downhole conditions.
U.S. Pat. No. 5,353,637 (the ""637 patent), describes multiple, axially spaced inflatable packers included as part of a wireline or coil tubing supported sonde that is used to conduct measurements in cased or uncased boreholes. The ""637 patent system measures conditions in the wellbore between axially spaced inflatable packers and sends the measurement values to the surface over the supporting wireline cable.
The ""945 patent, previously noted, describes methods and apparatus for early evaluation testing of subsurface formation. A drill-string-supported assembly that includes one or more well packers and measuring instruments is used to measure subsurface pressures. Recorded measurements are accessed by retrieval of the drill string or connection with a wireline coupling. The system may also provide constant remote communication with the surface through mud pulse telemetry.
The present invention provides methods and apparatus for directly measuring a subsurface well condition, transmitting the measured condition values to the well surface using FPT, and evaluating the transmitted data to determine the value of a well condition at a location in the well remote from the well surface.
One method of the present invention measures a subsurface pressure directly while the circulating fluid system is off, records the measured values, transmits the recorded pressure values to the well surface when circulation is resumed using FPT, and evaluates the received data to determine such conditions as casing cement integrity, kick tolerance of a newly drilled borehole section, openhole fracture strength, and formation pressure.
The method of the present invention is employed to determine surge and swab pressures by measuring and recording xe2x80x9cpumps offxe2x80x9d pressure changes caused by pipe movement and fluid flow rate increases. The measured values are recorded while the pumps are off and transmitted to the well surface when circulation is resumed using FPT. The received data are employed to adjust the speed of pipe movement or the rate of pumping to maintain well fluid pressures at optimum values as the pipe is being pulled or run and/or as the pumps are being started back up after a period of xe2x80x9cpumps off.xe2x80x9d
The methods of the invention are also employed to determine subsurface mud weight, cuttings, volumes, and other solids content of the well fluid, and to determine an equivalent circulating mud density.
In one method of the invention, measurements made while the fluid system of the well is circulating, or not, are taken at axially spaced locations in the wellbore to detect a pressure differential. Measurements taken with the pumps off are recorded. The measurement data are sent to the well surface using FPT. Circulating pressure measurements are recorded or are transmitted to the surface as they are taken using FPT. The received data are used to detect the occurrence of a kick or to monitor mud rheology or solids content of the circulating mud. Circulating and non-circulating measurements are used to determine the pressure effect of circulation on the wellbore.
The present invention also employs a method of directly measuring subsurface well conditions in an area of the wellbore that is temporarily freed from the effects of circulating well fluids to obtain true subsurface condition values. Where the area being measured is isolated from the circulating fluid by an isolation packer during xe2x80x9cpumps on,xe2x80x9d the measured data may be transmitted real time through the circulating fluid using FPT. In another method of the invention, measurements are made in an isolated part of the wellbore, the measurements are recorded, contact with the circulating well fluid is reestablished, and the recorded data is transmitted to the well surface using FPT. In either application, conventional FPT systems may be employed in a pumps off condition and/or in combination with an isolating well packer and subsurface recorder and measuring devices to obtain direct measurement of subsurface well parameters free of the effects of the well fluid used in the well""s circulation system.
The apparatus of the invention comprises a drill-string-carried assembly that is employed to perform MWD measurements, as well as to selectively isolate the subsurface well area to be evaluated. The preferred form of the invention includes two axially spaced inflatable well packers, either one of which, or both, may be used to isolate a section of the wellbore. The assembly is equipped with axially spaced measuring instruments, recording equipment, a fluid receiving reservoir, valves, and control equipment that may be actuated from the well surface.
The apparatus may be used to directly measure the swab and surge pressures caused by drill string movement, the surge pressure caused by the initiation of fluid circulation, the formation strength, the formation pressure, the downhole fluid density, the effectiveness of kill fluids being added to the circulation system and other subsurface variables related to the condition of the well. Data measured and/or recorded at the subsurface location are sent by FPT to the well surface through the circulating well fluid.
The apparatus of the present invention is the provided with axially spaced sensors, such as PWD sensors or temperature sensors, to provide simultaneous measurement of wellbore conditions at axially spaced locations either with the packers set or unset. The differential in the spaced measurements is used to evaluate subsurface wellbore conditions. The measured values may be transmitted to the well surface as they are being taken using FPT, or they may be taken in a static or isolated area of the well fluid and recorded for subsequent transmission using FPT when communication with circulating fluid is reestablished.
From the foregoing, it will be appreciated that a primary object of the present invention is to measure and record subsurface well conditions within an area of the wellbore, free from the effects of fluid circulating in the circulation system of the well, and transmit the recorded data to the well surface using FPT for directly evaluating one or more subsurface conditions without having to correct for the effects of the circulating well fluids.
Another object of the present invention is to provide an apparatus carried by the drill string that may be employed to isolate a section of the wellbore with one or more inflatable packers, measure, and record variable well conditions within the isolated section, and transmit the recorded data to the well surface using FPT.
Yet another object of the present invention is to provide a method of directly measuring subsurface pressure, temperature, and/or other variables within a wellbore at axially spaced positions within the wellbore to obtain differential values of such variables and transmitting the measured values to the well surface using FPT while the pumps are on or after circulation of the well fluids is reestablished.
Yet another object of the present invention is to provide a method for directly measuring the effects of pressure changes induced in a wellbore due to the movement of the drilling string assembly within the wellbore, to record the changes, and to transmit the recorded data through the well fluids using FPT.
An important object of the present invention is to provide a drill-string-carried tool having provision to isolate a section of a wellbore from the well fluids in the bore, receive formation fluids in a reservoir chamber included in the well tool and measure variable parameters of the entry of such formation fluids into the chamber, record such measurements, and subsequently transmit the recorded measurements to the well surface using FPT.
An object of the present invention is to provide a drill-string-supported assembly that can isolate a section of a wellbore, receive fluids from the formation in the isolated section of the wellbore, measure variable characteristics regarding the fluid being received from the formation, record such measured characteristics, and subsequently transmit the recorded characteristics to the well surface using FPT.
Another object of the present invention is to provide a subsurface assembly included as part of a drilling string assembly for isolating a section of a wellbore from the circulating fluids within the well, such assembly having expandable packer seals that are normally protected within a wear protecting sleeve that may be displaced from the packer seal to permit engagement of the seal with the surrounding formation.
It is an object of the present invention to provide a composite subsurface tool, carried by a drill string and included as part of a drilling assembly comprising dual, axially spaced inflatable packers that can be expanded radially to seal off the wellbore area between the packers, protective covering over the packers that is displaced when the packers are to be expanded, a circulating sub above the uppermost packer for circulating well fluids while an area of the wellbore is isolated, a receiving chamber for accepting fluid flow from the formation in the isolated wellbore area, an FPT module for conveying data to the well surface through the circulating well fluids, a measurement system for measuring wellbore conditions, a recording system for recording measured values, and a self-contained control system responsive to well surface commands for initiating setting and release of the well packers and for controlling the taking, recording, and transmission of measurement values.